Borrowings Under Credit Facilities and Related Items (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017		Mar. 31, 2016		Dec. 31, 2016
Aggregate borrowings under the unsecured line of credit arrangements
Balance outstanding at quarter end	$ 522,000	[1]	$ 645,000	[1]	$ 645,000
Maximum amount outstanding at any month end	1,010,000		945,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 796,356		$ 671,044
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	1.81%		1.29%

[1] As of March 31, 2017, letters of credit in the aggregate amount of $32,456,000 have been issued, which reduces the borrowing capacity on the unsecured revolving credit facility.